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Esr Study of Solutions of Sulfur and Polysulfides in Liquid Ammonia V

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Esr Study of Solutions of Sulfur and Polysulfides in Liquid Ammonia V ESR STUDY OF SOLUTIONS OF SULFUR AND POLYSULFIDES IN LIQUID AMMONIA V. Pinon, E. Levillain, A. Demortier, J. Lelieur To cite this version: V. Pinon, E. Levillain, A. Demortier, J. Lelieur. ESR STUDY OF SOLUTIONS OF SULFUR AND POLYSULFIDES IN LIQUID AMMONIA. Journal de Physique IV Proceedings, EDP Sciences, 1991, 01 (C5), pp.C5-223-C5-230. 10.1051/jp4:1991526. jpa-00250650 HAL Id: jpa-00250650 https://hal.archives-ouvertes.fr/jpa-00250650 Submitted on 1 Jan 1991 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE IV Colloque C5, supplement au Journal de Physique I, Vol. 1, decembre 1991 C 5 - 2 2 3 ESR STUDY OF SOLUTIONS OF SULFUR AND POLYSULFIDES IN LIQUID AMMONIA V. PINON, E. LEVILLAIN, A. DEMORTIER and J.P. LELIEUR URA 253 CNRS, HEI, 13 rue de Toul, F-59046 Lille cedex, France Résumé : L'étude RPE des solutions de soufre et de polysulfures dans l'ammoniac liquide montre la présence d'une seule raie lorentzienne, située à g = 2.0292. Ce signal doit être attribué au radical anion S3" identifié dans ces solutions par spectrophotométrie et par spectroscopie Raman, et qui est donc la seule espèce radicalaire dans ces solutions. La concentration du radical S3" a été déduite des expériences RPE pour une large gamme de concentration des solutions. Pour les différentes solutions étudiées, la concentration de S3" augmente avec la concentration de la solution et passe par un maximum observé pour des solutions concentrées et attribué qualitativement à un appartement des spins des radicaux S3". Abstract : An ESR study of solutions of sulfur and polysulfides in liquid ammonia gives evidence of a simple Lorentzian line located at g = 2.0292. This signal must be assigned to the radical anion S3" identified in these solutions by spectrophotometry and Raman spectroscopy. It is shown that S3" is the only radical species in these solutions. The concentration of S3" has been determined from the ESR experiments for a wide concentration range of the solutions. For the various solutions, the concentration of S3" increases with the concentration of the solution and goes through a maximum, located in the concentrated range of solutions. This maximum is qualitatively interpreted by a spin pairing mechanism between the S3" radicals. Introduction A significant step in the understanding of the solutions of sulfur in liquid ammonia (SAS) was the identification in these solutions of S4N" and of the radical anion S3" by Chivers and Lau III using Raman spectroscopy. However an ESR signal had not been previously observed in these solutions 111. Chivers and Lau attributed the lack of an ESR signal in SAS to a possible dimerization of the S3" radical. The identification of S3" in SAS was confirmed by Bernard et al. /3,4/ using Raman spectroscopy and 2 spectrophotometry. Dubois et al. 75,6/ showed that S6 ~ is the least reduced polysulfide in liquid Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1991526 C5-224 JOURNAL DE PHYSIQUE IV ammonia, and it is partly dissociated into S3-. The dissociation of S62- is observed only for temperatures higher than ca. 200 K, and is strongly temperature dependent. Dubois et al. /5,6/ also observed that polysulfides are disproportionated in liquid ammonia, and that the disproportionation is higher for acidic solutions. In neutral solutions (Li2S,), the S3- radical has been observed from Raman spectroscopy and from spectrophotometry for n > 3. It is not observed for Li2S3-NH3 solutions (for instance) because S32- is not disproportionated in neutral solutions. However, in acidic solutions ,(NH4)2Sn ,the S3- radical has been observed for n > 1. Such differences result from the different disproportionation in neutral and acidic solutions. The purpose of the present paper is to report the ESR identification of S3- in the various ammonia solutions in which this radical has been previously observed with other techniques : sulfur, lithium polysulfides and ammonium polysulfides. The purpose of these ESR experiments was also to check if S3- is the only radical in the investigated solutions, and also to determine the concentration of S3- in rather concentrated solutions, because for these highly colored solutions the concentration of S3- can only be deduced from the absorption spectra for dilute solutions. Experimental The lithium and ammonium polysulfide solutions are prepared by reducing sulfur with lithium and hydrogen suEde respectively. The preparation of the solutions /5,6/ , the ESR cells and the experimental conditions 1131 have been previously described. 9 ,8.. Figure 1. ESR signal of S3- at 290 K in a Li2S6 - NH3 solution (4 10-3 M). The experimental signal is 0 0 0 0 parabolicfitted by baseline.the sum of a Lorentzian lineshape and a :< I 9~ .: . :experimental data points. o o o : Lorentzian lineshape :parabolic baseline t~.~~~~.~.14.4~~~~8ul~~u~~~*.~t~~~8~ 1500 2500 9500 4500 The sensitivity on the vertical scale is four times MAGNETIC FIELD (GAUSS) larger for the residuals than for the ESR signal. The experimental signal is always rather weak and this leads to use high receiver gains. In these conditions a drift of the baseline is observed. The parameters of the ESR lineshape have been determined by fitting the experimental signal (Fig. 1) to the sum of a parabolic curve describing the drift of the baseline and of a Lorentzian signal following 171 : where H, is the magnetic field at the center of the line, AHpp the peak-to-peak linewidth of the derivative, Y the maximun of the derivative ;a, b and c are the parameters describing the parabolic shape of the baseline. The fit of f(H) to the experimental signal using a non-linear least squares technique allows the determination of H,, AHpp, Y, a, b and c. These parameters allow the calculation of the area A, of the absorption signal using the classical formula I71 relative to a Lorentzian lineshape : The determination of the area A, was found more accurate with the use of this fit procedure and Eq.(2) than with the numerical double integration technique available from the Bruker ESP 1600 computer program. The numerical double integration technique leads systematically to lower values for A, by ca.5 %. This is assigned to the broad linewidth of the Lorentzian lineshape. On the wings of the signal, the signal is merged in the noise over a large magnetic field range. Figure 2. Plot of A, versus A. The area A, is given for an internal diameter of 1 mm and the absorbance A for an optical pathlength of 0.05 cm. ABSORBANCE The conversion of the ESR signal of a given sample into the concentration of the paramagnetic species requires the calibration of the sensitivity of the spectrometer. For this purpose, we have demonstrated that the ESR signal of the Sg- radical anion observed in solution of lithium hexasulfde (Li2S6) in liquid ammonia can be used as a standard 181. It can easily be shown I81 that the area A, of the absorption signal is related to the absorbance A at 610 nm by : C5-226 JOURNAL DE PHYSIQUE IV where E is the extinction coefficient of S3- ,1 the optical path length, V the volume of sample undergoing the ESR resonance phenomenon, and P is a proportionality constant Consequently, if for samples at a given temperature, or for a sample at various temperature, A, is plotted versus the absorbance A at 610 nm, a linear variation must be obtained. Such a linear variation is found as shown in Fig. 2. Results. General asDea For all the investigated samples (sulfur and polysulfides in ammonia) a single ESR line has been observed for temperatures higher than 200 K. All the recorded lines can be fitted to a Lorentzian lineshape. The position of the line (g = 2.0292 +. 0.0009) is temperature and concentration independent. This ESR signal must be assigned to S3- because this radical has previously been identified in these solutions with Raman spectroscopy and visible spectrophotometry. For all the investigated samples, the area A, of the ESR signal, i.e. the concentration of S3-, decreases when the temperature decreases and the ESR signal is not detectable at 200 K (Figs. 3a and 3b). These observations are in agreement with the variations of the concentration of S3- with temperature observed by spectrophotometry in dilute solutions of sulfur or polysulfides in liquid ammonia. Figure 3. ESR signal of S3- ;A : in a S - NH3 solution (0.1 M) ;T= 286,255 , 225 K. ;B : in a Li2S6 solution (4 10-3 M) ; T = 290,280,270, ... 200 K. Solutions of sulfur in ammonia, The ESR signal has been detected in S-NH3 in a large concentration range from about 1 10-3 M up to saturated solution (3.6 M) between 200 and 290 K. At a given temperature, the variation of the concentration of S3- with the concentration of the solution displays an unexpected behavior (Fig. 4). In the dilute range of concentration, the concentration of S3- increases slowly with the concentration of the solution : for a 2.5 10-3 M solution, the concentration of Sg- is 3 10-4 M at room temperature. When the concentration of the solution is increased by a factor of one hundred (0.25 M), the concentration of S3- is increased by a factor of four and is close to 1.2 10-3 M.
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